Python Thread虚假唤醒概念与防范详解_Python

什么是虚假唤醒

虚假唤醒是一种现象，它只会出现在多线程环境中，指的是在多线程环境下，多个线程等待在同一个条件上，等到条件满足时，所有等待的线程都被唤醒，但由于多个线程执行的顺序不同，后面竞争到锁的线程在获得时间片时条件已经不再满足，线程应该继续睡眠但是却继续往下运行的一种现象。

上面是比较书面化的定义，我们用人能听懂的话来介绍一下虚假唤醒。

多线程环境的编程中，我们经常遇到让多个线程等待在一个条件上，等到这个条件成立的时候我们再去唤醒这些线程，让它们接着往下执行代码的场景。假如某一时刻条件成立，所有的线程都被唤醒了，然后去竞争锁，因为同一时刻只会有一个线程能拿到锁，其他的线程都会阻塞到锁上无法往下执行，等到成功争抢到锁的线程消费完条件，释放了锁，后面的线程继续运行，拿到锁时这个条件很可能已经不满足了，这个时候线程应该继续在这个条件上阻塞下去，而不应该继续执行，如果继续执行了，就说发生了虚假唤醒。

import threading
from threading import Condition
class Data:
  def __init__(self, cond, num):
      self.num = num
      self.cond = cond
  def add(self):
      self.cond: Condition = self.cond
      self.cond.acquire()
      if self.num > 0:
          self.cond.wait()
      self.num += 1
      print(threading.current_thread().getName(), self.num)
      self.cond.notifyAll()
      self.cond.release()
  def decr(self):
      self.cond: Condition = self.cond
      self.cond.acquire()
      if self.num == 0:
          self.cond.wait()
      self.num -= 1
      print(threading.current_thread().getName(), self.num)
      self.cond.notifyAll()
      self.cond.release()
if __name__ == '__main__':
  cond = Condition()
  num = 0
  data = Data(cond, 0)
  thread_add = threading.Thread(name="A", target=data.add)
  thread_decr = threading.Thread(name="B", target=data.decr)
  thread_add.start()
  thread_decr.start()

Python Thread虚假唤醒概念与防范详解

现在改用4个线程

import threading
from threading import Condition
class Data:
  def __init__(self, cond, num):
      self.num = num
      self.cond = cond
  def add(self):
      self.cond: Condition = self.cond
      self.cond.acquire()
      if self.num > 0:
          self.cond.wait()
      self.num += 1
      print(threading.current_thread().getName(), self.num)
      self.cond.notifyAll()
      self.cond.release()
  def decr(self):
      self.cond: Condition = self.cond
      self.cond.acquire()
      if self.num == 0:
          self.cond.wait()
      self.num -= 1
      print(threading.current_thread().getName(), self.num)
      self.cond.notifyAll()
      self.cond.release()
if __name__ == '__main__':
  cond = Condition()
  num = 0
  data = Data(cond, 0)
  thread_add = threading.Thread(name="A", target=data.add)
  thread_decr = threading.Thread(name="B", target=data.decr)
  thread_add2 = threading.Thread(name="C", target=data.add)
  thread_decr2 = threading.Thread(name="D", target=data.decr)
  thread_add.start()
  thread_decr.start()
  thread_add2.start()
  thread_decr2.start()

Python Thread虚假唤醒概念与防范详解

还没有出现问题！！！

使用20个线程同时跑

import threading
from threading import Condition
class Data:
  def __init__(self, cond, num):
      self.num = num
      self.cond = cond
  def add(self):
      self.cond: Condition = self.cond
      self.cond.acquire()
      if self.num > 0:
          self.cond.wait()
      self.num += 1
      print(threading.current_thread().getName(), self.num)
      self.cond.notifyAll()
      self.cond.release()
  def decr(self):
      self.cond: Condition = self.cond
      self.cond.acquire()
      if self.num == 0:
          self.cond.wait()
      self.num -= 1
      print(threading.current_thread().getName(), self.num)
      self.cond.notifyAll()
      self.cond.release()
if __name__ == '__main__':
  cond = Condition()
  num = 0
  data = Data(cond, 0)
  for i in range(10):
      thread_add = threading.Thread(name="A", target=data.add)
      thread_add.start()
  for i in range(10):
      thread_decr = threading.Thread(name="B", target=data.decr)
      thread_decr.start()

Python Thread虚假唤醒概念与防范详解

这时就出现了问题！！！

现在改用while进行判断

防止虚假唤醒：

import threading
from threading import Condition
class Data:
  def __init__(self, cond, num):
      self.num = num
      self.cond = cond
  def add(self):
      self.cond: Condition = self.cond
      self.cond.acquire()
      # 这里采用了while进行判断，防止虚假唤醒
      while self.num > 0:
          self.cond.wait()
      self.num += 1
      print(threading.current_thread().getName(), self.num)
      self.cond.notifyAll()
      self.cond.release()
  def decr(self):
      self.cond: Condition = self.cond
      self.cond.acquire()
      # 这里采用了while进行判断，防止虚假唤醒
      while self.num == 0:
          self.cond.wait()
      self.num -= 1
      print(threading.current_thread().getName(), self.num)
      self.cond.notifyAll()
      self.cond.release()
if __name__ == '__main__':
  cond = Condition()
  num = 0
  data = Data(cond, 0)
  for i in range(10):
      thread_add = threading.Thread(name="A", target=data.add)
      thread_add.start()
  for i in range(10):
      thread_decr = threading.Thread(name="B", target=data.decr)
      thread_decr.start()

Python Thread虚假唤醒概念与防范详解